Known conventional optical heads include optical heads configured in such a manner that a cylindrical lens and a lens holder are integrated together but the cylindrical lens and a light detector are separated from each other (for instance, Patent Document 1).
FIG. 27 and FIG. 28 illustrate an optical head, and a light-detection section thereof, disclosed in Patent Document 1.
In FIG. 27, a beam of light emitted by a semiconductor laser 101 is split into a plurality of different light beams by a diffraction grating 102. The light beams passing through the diffraction grating 102 are reflected by a beam splitter 103, and are converted into parallel light beams by a collimator lens 104. These light beams enter an objective lens 105 and become so-called three-beam converging light that is irradiated onto an optical disk 201. An objective lens actuator 106 drives the objective lens 105 in the optical axis direction (focus direction) and the radial direction of the optical disk 201. The beams of light reflected and diffracted by an information layer 202 of the optical disk 201 pass again through the objective lens 105, and then through the beam splitter 103. The light beams that traverse the beam splitter 103 pass through a cylindrical lens 115, and is incident on a light detector 120.
FIG. 28 is a partial schematic diagram of an optical head 200. An optical base 113 holds the semiconductor laser 101, the diffraction grating 102, the beam splitter 103, the collimator lens 104 and an objective lens actuator 106. The cylindrical lens 115 is disposed in such a manner that a concave cylindrical lens surface thereof, having negative power (concave lens effect), is on the side of the light detector. The position of the cylindrical lens 115 in the optical axis direction can be adjusted along the optical base 113 in a state where the cylindrical lens 115 is fixed to the lens holder 159. The cylindrical lens 115 is held in this state by an external jig. The position of the light detector 120 can be adjusted within a plane (X-Y plane) that is perpendicular to the optical axis, in a state where the light detector 120 is held by an external jig.
FIG. 29 illustrates schematically a light-receiving surface 121 of the light detector 120. A light beam that transmits the cylindrical lens 115 is received at a four-quadrant light-receiving region 140. The differences between the sum signals of diagonally-opposing regions within the four-quadrant light-receiving region 140 are computed to detect thereby a so-called focus signal. An RF signal is also detected by computing the sum signals from the four-quadrant light-receiving region 140.
A push-pull signal resulting from computing the signals obtained from the four-quadrant light-receiving region 140, plus signals corresponding to the quantity of light that is received at sub-beam light-receiving regions 141, are computed by a summing amplifier 144 and a differential amplifier 145. The three-beam method (so-called DPP method) yields a tracking error signal on the basis of which tracking servo is carried out in order to cause the objective lens 105 to follow a track in the information layer 202.
In order to secure symmetry and linearity in the focus error signal, the light detector 120 is disposed spaced apart from the concave cylindrical lens surface of the cylindrical lens 115. To that end, the position of the light detector 120, alone or together with the holder, is adjusted within the X-Y plane. This position adjustment is carried out while referring to the detection signal from the light detector 120, in such a manner that a light beam enters substantially the center of the four-quadrant light-receiving section 140. Thereafter, the light detector 120 (or holder) is fixed to the optical base 113.
The lens holder 159 to which the cylindrical lens 115 is fixed is held in a state whereby the lens holder 159 can move over the optical base 113 in the optical axis direction. The relative position with respect to the light detector 120 is adjusted by adjusting the cylindrical lens 115 in direction Z. The optical base 113 and a lens holder 159 are then fixed together. Through this adjustment in direction Z, the objective lens 105 and the information layer 202 become positioned at a just focus distance, and the focus error signal offset is cancelled. Specifically, the output of the focus error signal at the just focus distance is 0.
Optical heads are expected to be developed wherein the optical head can support recording or reproduction to/from small, high recording-density multilayer optical disks having two or more recording layers.
In order to support recording and reproduction to/from high recording-density multilayer optical disks, and to reduce the size of the optical head, large negative lens power is required to be achieved by forming a concave lens surface, having a small radius of curvature, on the cylindrical lens.
In an optical system having an optical head, however, it is impossible to avoid small errors during adjustment of the position of the light detector in directions X-Y and during adjustment of the position of the cylindrical lens in direction Z. As a result, a relative positional error between the light detector and the cylindrical lens arises on account of the positional offset of the cylindrical lens in direction Z and directions X-Y. This gives rise in turn to error in the position of the cylindrical lens in direction Z.
When such an optical system is fitted with a cylindrical lens having high lens power, such as the one described above, magnification by the detection optical system varies significantly depending on the relative positional error, and sub-beams may bear away from the sub-beam light-receiving regions of the light detector. In the light detector, angle error and positional error, in directions X-Y, with respect to the optical axis, cause the sub-beams to bear away, by a greater distance, from the sub-beam light-receiving regions, and give rise to significant deterioration of the quality of the tracking error signal, which may impair recording and reproduction performance. Thus, reducing the size of detection optical systems equipped with high-power cylindrical lenses is extremely difficult in optical systems having conventional optical heads.    Patent Document 1: JP 10-003683 A